Modern medicine has provided miraculous advances in effectively treating cancers that were previously a death sentence. In point of fact, lung cancer, breast cancer, melanoma, leukemia, and prostate cancer, among others, have seen significant improved in patient outcomes. One of the cancers that has not benefited from these advancements is pancreatic ductal adenocarcinoma (PDAC). Less than 10% of PDAC patients survive for five years after diagnosis, partly due to a lack of early detection strategies, while in diseases such as breast cancer early detection has vastly enhanced patient outcomes. PDAC has been underfunded and understudied; for instance, among the 73 cancer cell lines that encompass the NCI60 library of cell lines, no lines are derived from PDAC. Moreover, standard chemotherapeutics, targeted therapeutics, and even novel immune checkpoint therapies, have failed to improve outcomes in the vast majority of PDAC patients. Moving forward, we posit that the field is in desperate need of novel therapeutics that account for PDAC’s genetic makeup and its unique tumor microenvironment.

The most common genetic defect in PDAC cells is the loss of p16 (which is caused by either a mutation or promoter hypermethylation in 98% of PDAC cases). This near ubiquitous PDAC mutation is targetable. Normally p16 acts to inhibit the cell cycle by inhibiting CDK4/6, and cells with a loss of p16 become dependent on the aberrant and constitutive activation of this pathway. CDK4/6 inhibitors which inhibit this pathway have been found to have increased efficacy in patients with p16 pathway alterations. We have demonstrated the activity of the potent CDK4/6 inhibitor abemaciclib both in vitro (where we demonstrated abemaciclib caused both senescence and apoptosis) and in vivo. Daily treatment in vivo not only caused a decrease in tumor growth (3.2 fold reduction), but also caused a reduction in Ki67 staining, phosphorylated retinoblastoma protein staining, and an increase in TUNNEL staining. Taken together, these results indicate that abemaciclib is acting through the intended target and re-activating retinoblastoma protein in PDAC cells in vivo, and further indicates that abemaciclib could induce apoptosis in p16 deficient tumors in vivo. As most therapeutics in PDAC are most effective in combination with other therapies, we screened for synergy between abemaciclib and a variety of compounds and demonstrated that abemaciclib was significantly more effective in combination with inhibitors of the pro-oncogenic proteins YAP1 and of HuR. Furthermore, after generating abemaciclib resistant cell lines, we demonstrated that both YAP1 and HuR inhibitors were able to target and break this acquired resistance.

The ability of HuR inhibition to break resistance to abemaciclib is not surprising, as HuR has been found to be critical for PDAC resistance to many extrinsic stressors such as chemotherapies, radiation and targeted agents. The pro-survival and stress response protein, HuR, has also been shown to be critical for PDAC resistance to the intrinsic stresses it faces as well. In fact, PDAC exists in a resource scarce microenvironment characterized by its dense fibrotic stroma, poor vasculature, hypoxia and hypoglycemia. These conditions help to activate HuR, which plays a central role in PDAC resistance to therapeutics by binding to and stabilizing pro-survival mRNA targets. Herein, we review the approaches being used to target HuR, namely by targeting expression, translocation, and the ability to bind target mRNAs. We also describe a novel screening assay for identifying small molecules and their ability to inhibit HuR. Using this novel assay, we found that the FDA approved drug Pyrvinium Pamoate (PP) is a potent inhibitor of HuR in PDAC. We further demonstrated that PP is effective in vitro and in vivo for the treatment of PDAC, and for the first time, demonstrated the efficacy of oral treatment with human grade PP. We determined through metabolomic and phosphoproteomic screening that PP was also effective at targeting mitochondrial pathways in PDAC and demonstrated that PP was able to inhibit mitochondrial oxidative phosphorylation in PDAC at nM concentrations. Furthermore, we demonstrated that both the hexokinase inhibitor, 2-deoxy glucose, and PP were more effective in low glucose, and the two agents synergized in PDAC. We have moved this project from the bench to the bedside, and have received IND approval for moving PP to the clinic to further explore PP as a novel therapeutic for PDAC patients. Overall, this work describes unique vulnerabilities in PDAC, and clinically relevant therapeutics aimed to target these vulnerabilities.

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